The extrinsic muscles of the tongue are extremely important in the articulation of both consonants and vowels. They participate in shaping the vocal tract for normal articulation. However, little is known regarding the neural control of these muscles; specifically the central nervous system transmitters that modulate the motoneurons innervating the muscles. This project will investigate the motocircuitry of these motoneurons, utilizing a double-labeling electron microscopic technique. The styloglossus, hyoglossus, or genioglossus muscle of the cat will be injected with a retrograde neuronal tracer, cholera toxin B conjugated to horseradish peroxidase (CTB-HRP). Two to three days later, animals will be perfused and histochemistry performed to locate the motoneurons labeled from each injection. Then immunohistochemistry will then be done to label processes containing substance P, serotonin transporter, tyrosine hydroxylase, calcitonin gene-related peptide, thyrotropin-releasing hormone, gamma-aminobutyric acid, or arginine-vasopressin. It is hypothesized that the different extrinsic muscles are innervated by motoneurons that are differentially innervated by these neurotransmitters, thus allowing the coordinated activity of the muscles to correctly shape the vocal tract for speech. Rational pharmacotherapy for a variety of dysarthrias could then be developed as a result of these studies.